In imaging energy filters for transmission electron microscopes, aberrations cause lateral resolution losses in the achromatic image plane and in the dispersion plane wherein the energy spectrum of the electrons, which are transmitted through the object, arises and therefore cause a loss of energy resolution. Such energy filters are shown, for example, in
U.S. Pat. No. 4,760,261 directed to a so-called alpha filter and in U.S. Pat. No. 4,740,704 directed to a so-called omega filter. The dominating aberrations are of a second rank. The aberrations are defined by eighteen linearly independent aberration coefficients. The defective focusings are then dependent quadratically or bilinearly from geometric parameters such as axis distance of the image points and irradiation aperture and dependent from deviations of the electron energy of the nominal energy.
It is known to correct the geometric errors of the second order (Seidel aberrations) in the achromatic image plane and in the dispersion plane with a symmetrical assembly of the filter and by a astute selection of the guide of the fundamental paths through the filter as well as the use of hexapole correctors. This is known from the work of H. Rose and D. Krahl in xe2x80x9cOptical Sciencexe2x80x9d, edited by L. Reimer, pages 43 to 149 (Energy Filtering Transmission Electron Microscopy) and from the dissertation of S. Lanio entitled xe2x80x9cOptimization of Imaging Energy Filters for the Analytic Electron Microscopyxe2x80x9d, TH Darmstadt (1986). Because of the hexapole correction, the chromatic aberrations simultaneously vanish in the achromatic image plane. These chromatic aberrations are dependent upon the distance from the axes of the image points and on the energy deviations. In contrast, the axial chromatic aberrations in the achromatic image plane and in the dispersion plane remain uncorrected. The influence of the chromatic aberration in the achromatic image plane on the lateral resolution also can be neglected if a relatively large energy window is selected because this chromatic aberration is relatively small. The component of the chromatic axial aberration error (which is effective in the dispersive direction) in the dispersion plane Cxcex3xcex3k effects, in combination with the energy dispersion D of the energy filter, an inclination of the focus plane of the energy loss spectrum by an angle "THgr" relative to the dispersion plane in accordance with the equation:
tan "THgr"=Cxcex3xcex3k/Cxcex3k
wherein: Cxcex3k. is the dispersion coefficient which results from D=Cxcex3k/E. This inclination of the focus plane operates disadvantageously for a parallel registration of energy spectra wherein the dispersion plane is imaged on a detector plane by means of a round lens or a lens combination. The detector plane can, for example, be a photoplate or a CCD camera. A deterioration of the energy resolution with increasing energy loss must be accepted as a consequence of the inclination of the focus plane relative to the dispersion plane because of the axial chromatic error. This effect occurs increasingly when relatively large image fields contribute to the energy spectrum. These energy fields are characterized by a large axis distance parameter xcex3. For this reason, it has already been determined in the above-mentioned article of H. Rose and D. Krahl that the axial chromatic aberration must be corrected for parallel registration of energy spectra. Specific suggestions for the correction of the axial chromatic aberration are, however, not given.
U.S. Pat. No. 5,448,063 discloses an omega filter wherein all geometric aberrations of the second order as well as all chromatic aberrations of the second rank, and therefore the axial chromatic aberration, are supposedly corrected by a total of nine hexapole correctors. The examples given here for the filters are, however, not stigmatic in the achromatic image plane, that is, the input image plane of the filter is imaged by the filter astigmatically into the achromatic image plane. The focus differences between the intercepts perpendicular and parallel to the magnetic dipole fields of the deflecting magnets amount for the filters to 26.4 mm and 32.7 mm, respectively. These filters are therefore not suitable for use as imaging filters wherein an energy filtered object image is imaged into the detector plane.
It is an object of the invention to provide an electron microscope having an imaging magnetic energy filter which is suitable for imaging energy filtered object images or diffraction images into a detector plane as well as for imaging the dispersion plane into a detector plane for a parallel registration of the energy spectrum.
The electron microscope of the invention defines an electron beam path to a detector plane and includes: an imaging magnetic energy filter including a plurality of magnetic deflecting systems arranged symmetrically to a center plane (M) and a plurality of hexapoles arranged between the magnetic deflecting systems; a projection system disposed along the electron beam path downstream of the energy filter; the energy filter defining a dispersion plane (DA) and an achromatic image plane (BA); a control unit operatively connected to the projection system for switching over the projection system for selectively imaging the dispersion plane (DA) and the achromatic image plane (BA) into the detector plane; and, the control unit also being operatively connected to the hexapoles for changing the excitation of at least some of the hexapoles at the same time as the switchover of the projection system.
The electron microscope according to the invention includes an imaging magnetic energy filter having magnetic deflection systems arranged symmetrically to the center plane. The energy filter images a first input plane (the input image plane) stigmatically and achromatically into a first output plane (the achromatic image plane). At the same time, the energy filter images a second input plane (the input diffraction plane) stigmatically and dispersively into a second output plane (the dispersion plane).
Viewed in beam direction, a projection system follows the energy filter and this projection system can selectively image the dispersion plane or the achromatic image plane magnified into a detector plane. When imaging the achromatic image plane into the detector plane, then either an energy filtered object registration takes place (namely when the object is imaged into the input image plane of the energy filter) or an energy filtered registration of the object diffraction diagram takes place (when the Fourier plane of the objective, which images the object, is imaged into the input image plane of the energy filter). With the imaging of the dispersion plane into the detector plane, a parallel registration of the energy spectrum of the electrons takes place. The energy spectrum results after the interaction with the object.
Hexapole correctors are provided at least between the deflection magnets of the energy filter. Additional hexapole correctors are also mounted in the input image plane and in the achromatic image plane. A switchover of the excitations of a part of the hexapole correctors takes place with a switchover between the imaging of the achromatic image plane and the imaging of the dispersion plane into the detector plane. When imaging the achromatic image plane, the hexapoles are usually so excited that all geometric errors of the second order (Seidel aberrations) are corrected in the achromatic image plane. In contrast, when imaging the dispersion plane into the detector plane, the hexapoles are so excited, that, in addition to a spherical aberration of the second order, also the axial chromatic aberration of the energy filter or of the system (made up of energy filters and projection systems) is corrected in the dispersion plane. In contrast, other geometric image errors of the second order are permitted in the achromatic image plane since these geometric image errors are not disturbing when imaging the dispersion plane into the detector plane in order to take an energy spectrum.
The invention is based on the realization that a correction of the axial chromatic aberration while simultaneously maintaining the correction of the spherical aberration of the second order Cxcex3xcex3k is possible without additional hexapoles with a corresponding change of the excitation of the hexapoles already available for a correction of the geometric errors of the second order. Although geometric image errors of the second order occur because of the changed hexapole excitations, it was however recognized that these geometric image errors do not constitute a disturbance when imaging the dispersion plane into the detector plane and therefore precisely in those cases wherein the axial chromatic aberration becomes disadvantageously noticeable. Compared to electron microscopes having energy filters wherein the axial chromatic aberration is uncorrected, the invention makes possible a significantly increased energy resolution for a parallel registration of energy loss spectra without causing additional structural complexity.
In addition to the axial chromatic aberration of the energy filter, also off-axis chromatic aberrations of the projection system arranged downstream of the energy filter contributes to the inclination of the focus plane in the dispersion plane. Depending upon the sign and magnitude of these off-axis chromatic aberrations of the projection system, the axial chromatic aberration of the energy filter can be overcorrected or be undercorrected via a corresponding excitation of the hexapoles so that the remaining residual of the axial chromatic aberration of the energy filter compensates the off-axis chromatic aberrations of the projection system.
The energy filter includes four magnetic deflecting regions in a specific embodiment of the invention. Two hexapoles each are provided between the first and second deflecting region and two further hexapoles are provided between the third and fourth deflection region. The two hexapoles between the third and fourth deflection region are symmetrical to the two hexapoles between the first and second deflecting regions. Furthermore, an additional hexapole is mounted in the symmetry plane of the energy filter. When switching over between the imaging of the dispersion plane and the imaging of the achromatic image plane, only the excitation of the following is changed: one of the two hexapoles between the first and second deflecting region, one of the hexapoles symmetrical thereto between the second and fourth deflecting region and the hexapole in the symmetry plane. In contrast, the excitations of all other hexapoles remain unchanged during the switchover. The excitation change of the hexapole between the first and second deflecting regions and of the hexapole symmetrical thereto between the third and fourth deflecting regions is opposite to the excitation change of the hexapole in the symmetry plane.
In the specific embodiment described, two further hexapoles are provided of which one is mounted in the achromatic image plane and the other is mounted in the input plane conjugated thereto (that is, the input image plane of the energy filter).
When imaging the dispersion plane into the detector plane, the excitation of the hexapole in the symmetry plane is greater by a factor of 2 to 4 (preferably by a factor of 2.81) than when imaging the achromatic image plane into the detector plane. In contrast thereto, the excitation of the first hexapole rearward of the first deflection region is greater when imaging the achromatic image plane into the detector plane by a factor of 1.5 to 2.5 (preferably, by a factor of 1.87) than when imaging the dispersion plane into the detector plane.